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R-Ras Regulation of Ca2+ Handling

Ras-like Ras (R-Ras), a member of the Ras family, has been implicated in both cell adhesion and apoptosis, as has intracellular Ca2+. Koopman et al. investigated the possible relationship between the two and obtained evidence suggesting that these effects of R-Ras may be mediated through alterations in intracellular Ca2+ handling. The authors expressed constitutively active (V38R-Ras) and dominant-negative (N43R-Ras) R-Ras mutants in CHO cells expressing a cholecystokinin (CCK) receptor. Cells expressing N43R-Ras were larger and rounder than cells expressing V38R-Ras, which were larger and rounder than controls. Inhibition of the sarcoplasmic and endoplasmic reticulum (ER) Ca2+-ATPase with thapsigargin (in the absence of extracellular Ca2+) transiently increased cytosolic Ca2+ concentration ([Ca2+]i, measured with indo-1 using high-speed confocal microscopy); V38R-Ras decreased the amplitude and shortened the duration of this Ca2+ transient. V38R-Ras also attenuated the inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ transient produced by 100 nM CCK (which releases the entire thapsigargin-sensitive store) and reduced the frequency of [Ca2+]i oscillations elicited by lower concentrations of CCK (in the presence of extracellular Ca2+). N43R-Ras did not affect the Ca2+ transients produced by thapsigargin or CCK. Neither mutant affected IP3 production. Analysis of the amplitude and kinetics of Ca2+ transients under various conditions suggested that activated R-Ras increased Ca2+ leakage from the ER and thereby reduced Ca2+ stores. The authors proposed that reduction in Ca2+ stores may underlie the antiapoptotic effect of R-Ras, whereas reduction in the frequency of stimulus-induced oscillations may be involved in R-Ras's effects on cell adhesion.